VII. ANALYTICAL METHODS FOR OZONE, PM, AND THEIR CHEMICAL PRECURSORS
Because of the heavy focus of the SOS research approach on observation-based methods of evaluation and development of Observation-Based Models (OBMs), it was also necessary to place heavy emphasis on the accuracy, precision, detection limits, stability, and field-worthiness of both the measurement instruments we used and scientist, engineers, graduate students and post-docs who calibrated, operated, and compared the performance and streams of measurement data produced by those instruments.
Thus, SOS gave significant attention to the selection and comparative evaluation of:
1) Instruments, methods, physical and chemical calibration standards, and operating protocols for direct measurements of ozone, PM, and their many precursors;
2) Direct and indirect methods for measurement of meteorological variables including horizontal and vertical wind speeds and directions, mixing height of the planetary boundary layer, air and soil temperatures, moisture content of soil, relative humidity, and their horizontal and vertical temporal and spatial variability;
- Optimizing the deployment of instruments at ground-based monitoring sites and in various sizes and types of fixed-wing aircraft, and helicopter-based, balloon-based, and tower- or tall-building-based measurement platforms;
- Integration, coordination, and deployment of personnel and measurement teams during carefully designed and intensive field measurement campaigns; and
- Analysis, interpretation, visual display, intercomparison, and both temporary and permanent archiving of measurement data.
As shown by the scientific findings listed below, these analytical methods-development and improvement efforts consumed a significant part of SOS' creative energies and financial resources.
A. Gas-Phase Methods (GPM)
GPM1. SOS developed a wide array of improved methods for measuring air concentrations of volatile organic compounds (VOC) from both human and natural sources (Lee et al., 1993; Lee and Zhou, 1993, 1994; Cantrell et al., 1993; Apel and Calvert, 1994; Das and Aneja, 1994a, 1994b; Farmer et al., 1994; Riemer et al., 1994; Lee et al., 1995, 1996; Fischer et al., 1995; Weinstein-Lloyd and Lee, 1995; Apel et al., 1994, 1995; Bernardo-Bricker et al., 1995; Buhr et al., 1995a; Gilpin et al., 1997; Alvarez et al., 1998; Apel et al., 1998a, 1998b; Daughtrey et al., 1998; Luke et al., 1998; Parrish et al., 1998; Tanner et al., 1998; Williams et al., 1998).
GPM2. Improved methods were developed for measuring oxygenated organic compounds, mainly aldehydes and alcohols (Lee and Zhou, 1993; Lee et al., 1995, 1996, 1998; Apel et al., 1998b) and for measuring hydrocarbons containing more than 10 carbon atoms (Zielinska et al., 1996).
GPM3. Based on studies first undertaken in Atlanta, Georgia and Nashville, Tennessee, in comparison with research-grade instruments, the commercially-available instruments commonly used for measuring NOx concentrations in air are: 1) not sensitive enough to measure NO reliably, and 2) are not specific enough to measure NO2 reliably (Fehsenfeld et al., 1998).
GPM4. Field inter-comparisons between flow-through and chamber techniques for measuring soil NO emissions showed that both methods yielded similar emission rate estimates (Valente et al., 1995).
GPM5. An inter-comparison between rawinsonde systems, radar acoustic sounding systems, and LIDAR systems for estimating the mixing height of the atmosphere showed that rawinsonde systems gave the most reliable estimates of mixing height especially during early morning hours in the Nashville area (Marsik et al., 1995).
GPM6. SOS developed a high-priority list of 60 VOC compounds for field studies in the SOS region and developed a standard mixture of 55 of these compounds for use in determining elution time standards for chromatograms. SOS also showed that aluminum canisters with interior walls coated with inorganic polymers had no detectable loss of sample integrity over the course of a year, thus indicating the viability of the canister method for quantitative field sampling and measurement of high-priority VOC concentrations (Apel and Calvert, 1994; Apel et al., 1994, 1998a; Bernardo-Bricker et al., 1995).
GPM7. SOS developed and evaluated a protocol for accurate and precise quantification of C2-C10 VOC that can be used in air-quality monitoring networks. This protocol was largely adopted in the network of Photochemical Assessment Monitoring Stations (PAMS) initiated by the USEPA in 1994 (Apel and Calvert, 1994; Apel et al., 1994, 1998a).
GPM8. Agreement among laboratories for VOC mixtures with concentrations of 1-30 ppbv was very good, provided high quality gas-phase standards are used and introduced in the measurement instrument in a similar manner as the air samples (Apel et al., 2000).
GPM9. Blind intercomparisons between different groups of investigators showed that the gas-chromatography elution-time technique can be used to quantify hydrocarbon species of interest with a precision of about 10% (Apel et al., 2000).
GPM10. Excellent agreement was found between measurements of ozone, NOx, NOz, NO, NO2, and HNO3 made in remote sensing aircraft, in-situ sensing aircraft, and different in-situ sensing aircraft flying side-by-side. Very good agreement also was found between ground-based and in-situ aircraft measurements for most of the chemical species listed above (Apel et al., 1998a, 1998b; Luke et al., 1998).
GPM11. A comprehensive intercomparison of ground-based NOy measurement systems employing both gold/CO and molybdenum catalytic reduction systems indicated good agreement between the two methods and among various analysts using both measurement methods (Williams et al., 1998).
GPM12. Despite very substantial agreement among ground-based NOy measurements, much poorer agreement was found among the aircraft and between the aircraft and ground-based measurements of NOy during the Nashville/Middle Tennessee Ozone Study (Luke et al., 1998).
GPM13. Rigorous intercomparison of simultaneous field measurements of ambient VOC by different techniques and researchers is critically important in determining the veracity of ambient air concentration measurements. SOS scientists developed a series of rigorous internal consistency tests for this purpose (Parrish et al., 1998).
GPM14. A CIMS (mass spectrometry with chemical ionization) instrument to measure gas-phase HNO3 was developed and demonstrated to be sensitive with fast response (detection limit of approximately 15 pptv for 1 s integration), accurate, precise, and interference-free. It was tested in ground-based intercomparisons at the Green Mountain Mesa field site in Boulder, CO (Fehsenfeld et al., 1998; Huey et al., 1998).
GPM15. CIMS techniques for measurement of HNO3, isoprene, and ammonia were developed and tested in ground-based intercomparisons during the Nashville/Middle Tennessee Ozone Study. These techniques promise to provide sensitive and fast aircraft measurements of those species. The selectivity of the isoprene technique must be tested by comparison to other techniques (Fehsenfeld et al., 1998).
GPM16. Several different instruments for measuring NOy were deployed and intercompared during the Nashville/Middle Tennessee Ozone Study. Results indicated that NOy can be measured reliably in urban and suburban environments with existing instrumentation (Williams et al., 1998).
GPM17. SOS made the first continuous OH and HO2 measurements in urban environments during the Nashville/Middle Tennessee Ozone Study. The measurements, when compared to models, test the fundamental atmospheric chemistry that underpins chemical transport models. For Nashville SOS' OH and HO2 measurements agreed to within a factor of two with model calculations near midday, but tend to be larger than models in the evening, at night, and for periods when nitrogen oxides are especially abundant. These observations indicate unidentified HOx sources and questions about HOx- NOx chemistry (Kovacs and Brune, 2000).
GPM18. Total OH loss-rate measurement (TOHLM) tests were used to determine the completeness of measured VOC inventories during the Nashville/Middle Tennessee Ozone Study. The presence of unmeasured VOC is indicated if TOHLM-measured OH loss rates are greater than those calculated from the sum of VOC measurements and OH reaction rate coefficients. Preliminary Nashville observations indicate that OH loss rates are about twice those calculated, suggesting unmeasured VOC (Kovacs and Brune, 2000).
GPM19. SOS took a leadership role in the atmospheric science community and has partnered with the NOAA Climate and Global Change Program in conducting "The Non Methane Hydrocarbon InterComparison Experiment" (NOMHICE). This experiment assessed the accuracy of analytical methods used to determine mixing ratios of atmospheric non-methane hydrocarbons (NMHC) (Apel et al., 2000, 2003).
GPM20. Based on a blind intercomparison of six ambient formaldehyde measurement techniques in 1995, SOS concluded that gas-phase standards should be employed with any of the measurement techniques, and the cartridge measurement methods were limited by long collection periods, and were generally lower in precision. Airborne CH2O measurements by two fast and sensitive techniques, tunable diode laser absorption spectroscopy (TDLAS) and coil/2,4-dinitrophenylhydrazine (CDNPH), indicated that, on average, both instruments measured identical ambient CH2O concentrations to better than 0.1-ppbv over the 0 to 0.8-ppbv-concentration range. However, significant differences, larger than the combined 2s total uncertainty estimates, were observed in 29% of data set. Careful attention must be paid to the behavior of CH2O in the inlet for accurate airborne measurements (Gilpin et al., 1997).
GPM21. SOS prepared the first quantified and verified carbonyl standards, which were prepared gravimetrically with both calibrated permeation sources and in specially treated aluminum cylinders. Techniques such as atomic emission detection (AED) and FTIR have been applied to verify the accuracy of these carbonyl standards (Apel et al., 1998a).
GPM22. Intercomparisons of cartridge-based (Si-Gel and C18) and GC-MS measurements of carbonyls by SOS investigators showed serious discrepancies, indicating that more research is needed to resolve these discrepancies (Apel et al., 1998b).
GPM23. A relatively fast-response (15-minute cycle) GC-FID technique was developed to measure carbonyls and other oxygenates aboard aircraft. A GC-MS technique was also being developed to measure carbonyls with a 5-minute time response (Apel et al., 1998b).
GPM24. A photolytic converter utilizing the focused UV output from a high-pressure mercury (Hg) arc lamp was developed and tested. The new configuration permitted simple and accurate retrieval of ambient NO2 data at very high time resolution, was more specific, provided increased sensitivity, and was less expensive to operate than previous photolytic converter designs (Ryerson et al., 2000).
GPM25. Rapid and quantitative sampling of NOy species, including HNO3, was demonstrated using a short, heated Teflon inlet. In flight, standard addition calibrations of HNO3 at the aircraft inlet demonstrated freedom from significant surface adsorption of HNO3, which has significantly compromised measurements through other aircraft inlets (Ryerson et al., 1999).
GPM26. Intercomparisons of ground-based NO2 and NOy measurements demonstrated that laser-induced fluorescence, differential optical absorption spectroscopy, and photolysis-chemiluminescence techniques are all capable of accurately quantifying atmospheric NO2 above 1 ppbv. Further, molybdenum oxide and gold converters were shown to be capable of accurately measuring NOy above 2 ppbv in both urban and suburban environments typical of the SOS region. These studies also concluded that generation of reliable NO2 or NOy data still demands skilled operators and dedicated, critical oversight during the measurement process (Williams et al., 1998).
GPM27. Gas chromatographic methods for peroxyacyl nitrates (PANs) were refined to provide rapid and sensitive measurements. Aircraft-based instrumentation was developed to measure four of the major compounds of interest - PAN, PPN, PiBN, and MPAN - every 3.5 minutes. The measurement of PANs by proton-transfer reaction mass spectrometry (PTR-MS) was deployed during the Nashville '99 Intensive (Hansel and Wisthaler, 2000). While still in the development stage, this method has the potential to provide rapid (10 sec) measurements of PAN aboard aircraft (Williams et al., 2000).
GPM28. Two different calibration methods for PAN were developed: 1) a diffusion source method, and 2) a method based on photochemical production of PAN in acetone/air/CO/NO mixtures. The diffusion system relies on a NOy measurement for calibration, while the photochemical source relies on a known, efficient conversion of an NO standard to PAN. The two methods were compared during the TexAQS 2000 study and were found to agree within 5% (Williams et al., 2000).
GPM29. An automated system for measurement of the organic nitrates produced from OH radical attack on isoprene was developed and deployed at the Dickson site during SOS' Nashville '99 study. These compounds are produced when the peroxy radicals derived from OH reaction with isoprene, reacts with NO to produce a set of 8 isomeric RONO2 species. The maximum concentrations of the sum of these species were in the 100-200 pptv range, much higher than observed in a previous study. Comparison of these two data sets provides a good opportunity to examine the NOx-dependence of this aspect of isoprene photochemistry (Grossenbacher et al., 2001).
GPM30. An instrument based on vacuum UV resonance fluorescence was developed that is capable of fast (~1 Hz), accurate (~5%), and precise (~1 ppbv) measurement of CO from an aircraft platform. Intercomparisons with other techniques demonstrate that this method is highly specific with no identified interferences (Holloway et al., 2000).
These scientific findings (GPM1-GPM30) underscore the comprehensiveness and success of the SOS analytical program. As a result, the data bases that resulted from the SOS field campaigns are of high quality and usefulness. It was also because of this success that EPA assigned to SOS the task of developing the "Atlanta PM Supersite Program" - a mostly analytical methods development and intercomparison project.
B. Particle-Phase Methods (PPM)
Most of SOS' analytical-method studies for PM were undertaken in connection with the Atlanta Supersite Program. The major objectives of the Atlanta Supersite Program were intercomparisons and development of PM measurement techniques, and the characterization of aerosols and aerosol patterns in the Atlanta region.
PPM1. During SOS' Atlanta Supersite Program, integrated filter methods showed good agreement for PM2.5 mass (most samplers within ±20 percent), and sulfate and ammonium (most samplers within 10 percent) (Solomon et al., 2003a).
PPM2. Larger discrepancies between methods were found among integrated filter methods for measurement of nitrate (±3 0 to 35 percent), possibly due to the low ambient concentrations of this ion in the Atlanta metropolitan area. Higher variability also was found for the organic (OC) and elemental (EC) carbonaceous fractions of PM2.5. For all integrated filter samplers the OC variability ranged between 35 and 45 percent. EC variability was also high between the different analytical methods used (Solomon et al., 2003a).
PPM3. Based on a range of studies utilizing the PC-BOSS sampler, designed to quantify fine particle composition including semi-volatile compounds, it was estimated that 10 percent to 50 percent of the fine particulate mass was not measured with the PM2.5 FRM sampler; this was attributed to the loss of semi-volatile organic material and ammonium nitrate during sampling (Eatough et al., 2003; Modey et al., 2001).
PPM4. Secondary organic aerosol, which comprised about 46 percent of the measured organic carbon, was from a combination of in situ photochemical production and transport of more aged secondary organic aerosol during the Atlanta Supersite Study. This conclusion is based on diurnal patterns and correlations with ozone and carbon monoxide and estimates of the fraction of organic carbon (OC) from secondary organic aerosol formation processes from mean 1-hour fine particle OC and elemental carbon (EC) data collected by new semi-continuous instrumentation deployed during the study (Lim and Turpin, 2002; Lim et al., 2003; Weber et al., 2003a).
PPM5. In spite of the large uncertainties inherent in measuring carbon-containing particulate matter, which is very complex in composition, and in utilizing different operational techniques for measurement, there was generally good agreement between measurement systems (Lim et al., 2003).
PPM6. Transient PM2.5 episodes in which particle mass rapidly rises and falls over a period of a few hours but which go undetected with traditional time-integrated measurements were ubiquitous during the Atlanta Supersite Project. Continuous highly time-resolved measurements of fine particle mass and chemical composition, revealed these transient episodes. Speciated composition data show that these events are driven by sudden increases of two specific aerosol chemical components that dominate at different times - carbonaceous events in the early morning, and sulfate events in late afternoon (Weber et al., 2003b).
Apart from providing insights into sources, the unique chemical nature of these transient events may have specific health effects that previous epidemiologic studies based on highly averaged aerosol data could not readily resolve.
PPM7. Partitioning between the gaseous and condensed phases was in reasonable agreement with predictions of the ISORROPIA thermodynamic equilibrium model. Application of this model to near real-time measurements of fine particle sulfate, nitrate, and ammonium, and gas phase ammonia and nitric acid showed good agreement, with an indication of potential bias in estimates of acidity/alkalinity (Zhang et al., 2002).
PPM8. During the Atlanta Supersite Project, particle sizes were measured most accurately with ATOFMS, RSMS-II, and AMS during a side-by-side comparison of four particle mass spectrometers. The RSMS-II system can obtain composition information on individual particles as small as 15 nm. The three systems that utilize laser desorption/ionization, (PALMS, ATOFMS, and RSMS-II), produced mass spectra that were qualitative and representative of individual particles. The AMS instrument, which uses a two-step volatilization on a heated surface and ionization by electron impact, produced quantitative results representative of the ensemble of particles measured (Drewnick et al., 2003).
PPM9. Single-particle positive ion classifications obtained for the Atlanta Supersite data by laser-based instruments were broadly consistent and revealed similar trends as a function of size for organic, sulfate, and mineral particles. The AMS, which is the most quantitative of the mass spectrometers compared, had nitrate to sulfate molar ratios that were highly correlated with those of the semi-continuous instruments discussed above. Based on insights from the Atlanta study, subsequent studies, such as those undertaken at the New York EPA Supersite in the summer of 2002 (PEMTACS), demonstrated the quantitative measurement capabilities of the AMS. Overall, the strength and primary focus of the laser-based instruments are their ability to find associations between chemical species in individual particles with high time resolution (Middlebrook et al., 2003).
PPM10. Particles measured during the Atlanta Supersite Project typically included a major mass peak with a density in the ~1.5 to 1.7 g cm-3 range at 3-6 percent relative humidity. These data agreed with calculated densities based on measured size-resolved composition within about 5 percent (McMurry et al., 2002)